Strain gages of the above type are known from British Patent (GB-PS) 728,606. A measuring grid in the form of a resistor meander having contact tabs or solder tabs at its ends, is photolithographically etched out of a very thin, rolled metal foil, whereby the metal foil is connected to a second metal foil. After the etching an insulating epoxy resin backing is applied to the first metal foil and the second metal foil is removed. Problems are encountered in the manufacture, handling and application of strain gages with an epoxy resin backing because the epoxy resin backing is brittle and thus prone to breaking. Using such conventional strain gages in connection with transducers for measuring forces requires an especially careful handling. The use of such transducers in connection with stress analysis is not conventional. These problems are also present in connection with foil strain gages having a phenolic resin backing.
Further, foil strain gages are known which comprise a glass fiber reinforced backing of epoxy resin or phenolic resin. The elastic characteristics of such foil strain gages are improved based on the characteristics of fiber composite materials. However, due to a non-uniform quality of the fiber reinforcing material and due to thickness variations and structural faults, there are substantial impairments in the characteristic values of force transducers equipped with such foil strain gages. Especially it is not possible to predetermine the creep characteristics and hence these characteristics cannot be easily controlled.
According to German Patent Publication 2,916,427, corresponding to U.S. Pat. No. 4,322,707 (Ort) a foil strain gage transducer is known wherein creep characteristics are individually compensated for each gage element, whereby special foil strain gages are used, so that the effort and expense is correspondingly substantial. Faults in the glass fibers, such as small knots, glass clumps, or soiling of the glass fibers lead to a substantial waste in the manufacture of such conventional foil strain gages.
In another type of foil strain gage according to German Patent Publications 2,902,242 and 2,902,244 (Ruecker et al.) both the backing and the so-called encapsulation forming together a matrix, are made of synthetic material namely polyimide, whereby either respective films are adhesively bonded to the measuring grid foil or the metal foil is coated with the resin which is then cured. The measuring grid is made of an alloy of gold, palladium, and vanadium. This technique provides very robust foil strain gages because the matrix material is very flexible and has a high tear resistance. Such foil strain gages are preferably used for stress analysis. However, these strain gages are not suitable for the construction of precision force transducers because they creep too much. In German Patent Publication 2,902,242 (Ruecker et al.) the backing of polyimide is covered on its surface opposite the measuring grid foil with a solderable layer.
All three types of the above mentioned foil strain gages have the drawback that they are sensitive to moisture because the matrix swells or shrinks in response to the humidity of the atmosphere. Resistance changes of the foil strain gages are the result, whereby the zero load output of the respective transducer is uncertain. This characteristic of the above strain gages alone is a substantial drawback. Additionally, it prevents a precise adjustment of the thermal zero shift (TC zero). Additionally, the creeping error of transducers equipped with such foil strain gages is temperature dependent. All these characteristics together lead to enormous difficulties in the development and production of precision force transducers.
With regard to the above mentioned German Patent 2,916,427 corresponding to U.S. Pat. No. 4,322,707 (Ort), it is possible to compensate for the creeping of the transducer, by using special foil strain gages. However, that approach resides substantially in displacing the creeping characteristic as a function of temperature parallel to the temperature axis. It has not been possible heretofore to influence the shape of the characteristic or curve itself in a purposeful manner.
German Patent Publication 2,916,425 (Utner et al.) discloses a foil strain gage and a method for its production, wherein a measuring grid resistor is bonded in a material locking bond to a thin synthetic material film. The material locking bond is accomplished by a vapor deposition or sputtering. The synthetic material film is polyimide because it is capable of withstanding soldering temperatures and other heat treatment temperatures.
Swiss Patent Publication 640,346 (Krempl et al.) discloses a measuring transducer comprising a multi-layer structure including at least one flexible piezoelectric film made of a monoaxially oriented polymer. A multitude of polymers having suitable piezoelectric characteristics are mentioned such as PVDF, PVF, PVC, PAN, PMMA, FEP, PE and others. No suggestions are made with regard to an improved, temperature independent creep characteristic.
The publication "msr", Berlin 31 (1988), Nr. 8, pages 345 to 347 by Hamann, discusses "Sensors With An Organic Component", whereby numerous polymers are mentioned in passing. Moisture sensors, gas sensors, field effect transistors for use as chemical analysis sensors, optical sensors, piezoelectric sensors for various purposes and biological sensors are discussed without any mention of solving creep problems.
Polyphenylenesulfide is known as such. Reference is made to a brochure entitled "PPS Film Torelina .RTM.", published by the firm Toray. Numerous characteristics and uses of PPS are mentioned in this brochure. The characteristics of PPS include a high temperature, chemical, and radiation resistance, and a high dielectric constant. These characteristics make PPS films suitable for for high temperature capacitors, flexible printed circuit boards and other purposes. The superior characteristics of PPS discovered by the present inventor, for use in foil strain gages and transducers equippped with such foil strain gages are conspicuously absent from the manufacturer's own brochure.
The manufacturer has not recognized the superior creeping characteristics of PPS film and there is no suggestion in the Toray brochure toward the present invention.
The market calls for cheaper foil strain gages and less expensive, yet improved force transducers. The requirements to be met by multi-range scales, for example, call for substantial improvements in the creep specifications, in the temperature response characteristic of the zero load output signal, in the linearity, and in the hysteresis. All of these requirements cannot be satisfied simultaneously by conventional foil strain gages. Further, conventional foil strain gages have the drawback that they cannot achieve a plurality of desirable characteristics simultaneously and equally well. When conventional strain gages have one good characteristic, other desirable characteristics are lacking and vice versa.